1. Field of the Invention
The invention relates to a tapped linear integrated resistor comprising a doped resistance body in a semiconductor substrate coated with a layer of insulating material having local apertures in which the metal end contacts and tapping contacts are provided on the surface of the resistance body.
2. DESCRIPTION of the Related Art
Tapped linear integrated resistors are generally known and may be used in integrated circuits as voltage dividers, for example for obtaining accurately known fractional voltages in A/D and D/A converters.
A thin vapour-deposited metal layer is generally used as a resistance body on which teappings are provided. However, in the conventional manufacturing processes it appears to be very difficult to manufacture such a layer with sufficient accuracy, resulting in a spread of the resistance occurring in the resistance body, which leads to non-linearity of the resistor tappings.
If a doped resistance zone is used, its manufacturing process can be monitored well enough so that the above-mentioned spread problems do not occur or hardly occur. But on the other hand the length of the doped resistance body must be small for sufficiently low resistivity of the resistor (and also for limiting the dimensions of the chip on which the resistor is arranged), so that the tapping contacts of the resistor must be arranged relatively close to one another.
In the known tapped diffusion resistors the tapping contacts are provided in the form of relatively narrow metal strips extending across the width of the diffused resistance zone. Due to the limited accuracy of the lithographic techniques with which these metal strips are provided on the resistance body, errors relating to both the dimensions and the position of the metal strips occur. Particularly if these strips must be arranged close to one another, these dimensioning and positioning errors result in a relatively large non-linearity in the ultimate tapping ratios. Moreover, the current through the resistance body will predominantly concentrate directly under the tapping contacts in a relatively small cross-section directly under and next to the relevant contact. The resultant inhomogeneity in the field distribution within the resistance body also contributes to non-linearity of the resistor. Practice has proved that the known doped resistors of this type do not comply with the relatively stringent linearity requirements imposed in many applications such as in A/D and D/A converters.